This invention relates generally to a method of monitoring the activity of a cathodic protection system installed on a metallic structure that is located underwater.
Cathodic protection is a method of combating corrosion of metals that are exposed to the action of water, such as in the case of offshore drilling and production platforms and pipelines. The purpose of cathodic protection is to reduce or eliminate oxygen-driven corrosion of steel or other metals in a given environment, such as seawater or saline mud. The corrosion of metal in an electrolyte at ambient temperature is an electrochemical process involving the flow of electrons in metals and ions in electrolytes. The corrosion can be controlled by the application of currents from an external source, for example, a generator or rectifier or from a galvanic sacrificial anode, which supplies all of the current for the electrochemical reduction of the corrodant oxygen by a source other than the corroding steel. If all of the current for the electrochemical reduction is supplied by the external source, the corrosion of the steel is stopped. The full protection of the structure is evidenced by the shift in potential of the steel from the initially freely-corroding potential of about -500 to -600 millivolts, versus a silver/silver chloride reference half cell, to values more negative than - 780 millivolts when the cathodic protection is applied.
Generally, galvanic cathodic protection is obtained on offshore structures by the installation of large numbers of sacrificial aluminum alloy anodes to the structure. The average current density provided by the cathodic protection system is extremely important in providing full protection of the structure and in providing an indication of the life span of the galvanic anodes. Prior art workers have designed galvanic cathodic protection systems with a predetermined current density provided by a predetermined number of sacrificial anodes, each of which is a predetermined weight and size. However, since prior art workers have not had any means by which the actual cathodic current density provided by the protection system could be measured, these systems have generally been designed conservatively to ensure that the proper degree of protection is attained; this results in the installation of more anodes than are actually necessary to provide the proper degree of protection. These additional sacrificial anodes are costly and add to both the weight and the wave loading of the structure. Consequently, the structure must be designed to carry the additional weight of the unnecessary anodes and to withstand the greater wave loading caused by the anodes and the larger structure which results in a considerable and unnecessary expense in the construction of the structure.
Therefore, it is an object of the present invention to provide an apparatus for monitoring a cathodic protection system on an offshore structure to provide an indication of the current density provided by the cathodic protection system.